1. Field of the Invention
In the field of electrophotographic copying, particularly in that area characterized as plain paper copying, a photoconductor comprising a photoconductive composition coated on a rigid or flexible substrate is uniformly electrostatically charged in the dark and then exposed by being illuminated in an image pattern in accordance with graphic material on an original document. The photoconductor becomes discharged in the areas exposed to the illumination but retains its electrostatic charge in the areas not exposed to illumination which areas correspond to the graphic material on the original document. An electrostatically attractable developing material is applied to the photoconductor, the developing material adhering to the charged areas of the photoconductor material but not to the uncharged areas, thereby resulting in a visible image of developing material which is then transferred to plain paper or other suitable substrate to become the ultimate copy. Any residual developing material remaining on the photoconductor is cleaned and the photoconductor is reused in the above manner for subsequent copies. Since the developing material is heat fusible, application of heat to the sheet of paper causes the developing material to melt and be fused into the paper so as to be permanently affixed thereto.
Since most developing materials used in plain paper electrophotographic copying are formed of thermoplastic materials which melt at fairly high temperature levels, it is typical practice to utilize a fusing device having a radiant energy source of heat which generates an extremely high temperature atmosphere in the area through which the sheet of paper passes. This extremely high temperature is necessary in view of the fact that the fusing of the developing material must take place while the sheet of paper is moving through the fuser and a given segment of developing material is exposed to the source of heat for only a brief period of time. Since the temperature of the atmosphere immediately adjacent the paper exceeds the ignition temperature of the paper, it is apparent that the paper will catch fire as a result of almost spontaneous combustion if the piece of paper should stop moving in the fuser. Potentially, this situation could be very dangerous since other parts of the copying machine could catch fire from the burning paper which is only partly enclosed with the fuser. Also, if a portion of the burning paper has already exited from the fuser, previous copies deposited in a copy paper collection tray could catch fire and possibly cause personal injury to the operator of the copying machine. Thus, it is readily apparent that a very serious problem exists with respect to utilization of this type of fuser in an electrophotographic copying machine in the event of any failure in the copying machine which results in an interruption of the movement of the copy sheet through the fuser.
2. The Prior Art
The above described problem has long been recognized, not only in the electrophotographic copying field, but even before that in the motion picture field.
U.S. Pat. No. 1,845,840 discloses a restricted passageway through which motion picture film is drawn while passing through a projector so that if the film should catch fire for any reason while passing through the projector, the fire will be choked for lack of oxygen within the confined space of the constricted area to prevent the fire from exiting from this space.
In the electrophotographic copying field, several approaches have been taken in the prior art to satisfactorily cope with the problem with fire in the fusing devices of various types of copying machines. For example, in U.S. Pat. No. 3,357,401 an air jet detector is utilized to detect the presence of a copy sheet exiting from the fusing device. In the event that the copy sheet jams in the fusing device and fails to exit therefrom, an electric circuit responsive to the combined effects of the air jet and a timer operates to energize a fire extinguishing system.
In U.S. Pat. No. 3,705,289, a bridge circuit is utilized to detect temperature in the fuser above a predetermined limit to automatically terminate the operation of the fuser while allowing a ventilator to continue to operate. The bridge circuit continously monitors the fuser to assure that the temperature remains within predetermined limits.
In U.S. Pat. No. 3,804,516, a similar type of electric bridge circuit is utilized to detect the presence of a fire in the fuser by measuring any variation in the temperature resulting from a burning sheet in order to generate an output signal to warn the operator of the existence of the fire in the machine.
In U.S. Pat. No. 3,748,088 a mechanical device is utilized to measure the velocity of the copy paper traveling through the fuser and any variation from a predetermined velocity is electronically sensed to activate appropriate controls to de-energize the source of heat in the fuser to prevent the copy sheet from catching fire.
In U.S. Pat. No. 3,778,222 a fusing apparatus includes a means for sensing the presence of fire, in response to which suitable mechanical means operate to enclose the passageway of the copy sheet through the fusing device thereby preventing spread of the fire from the fusing device.
Most recently in U.S. Pat. No. 3,979,161 a fusing apparatus in an electrophotographic copier includes a fire extinguishing snuffer device located somewhat downstream from the heat radiating portion of the fuser so that in the event of combustion of the copy sheet the flames will be extinguished by the copy sheet passing between closely spaced plates of the snuffing device.
From the variety of approaches to the problem of copy paper combustion in the fusing apparatus of electrophotographic copiers as disclosed in the foregoing patents, it is apparent that this problem has not only been long recognized but also has received considerable attention in the electrophotographic copying industry. While all of the above approaches have obvious merit, in one way or another, in solving a serious and potentially hazardous problem inherent in electrophotographic copying machines, they are, nevertheless, all directed to the particular solution of detecting the presence of a fire and either warning of, extinguishing or confining the fire as the case may be. None of the approaches described above are directed toward the problem of preventing the fire from occurring in the first instance. Although some of the above described patents disclose monitoring systems for shutting down the fusing apparatus in the event of a malfunction causing excessive heat, which may be broadly considered as an attempt to prevent the occurrence of a fire, nevertheless no provision is made in these inventions to guard against an excessive condition which may result in a fire irrespective of the deactivation of the fusing system.